High molecular weight, normally solid interpolymers of ethylene and unsaturated carboxylic acids, such as acrylic acid and methacrylic acid, are well known. This present disclosure pertains to such interpolymers when made in a continuous manner under steady state conditions in well-stirred reactors at high pressure and elevated temperature using a free-radical type initiator whereby random, homogeneous interpolymers are made which are substantially of uniform composition, in contradistinction to polymers made under non-steady state conditions or in non-stirred tubular reactors or in batch reactions or in non-telomerized reactions, and in contradistinction to block copolymers or graft copolymers. In particular, the present disclosure pertains especially to high molecular weight random interpolymers of ethylene and unsaturated carboxylic acids with improved extrusion stability.
Patents which disclose interpolymerizations of ethylene and unsaturated carboxylic acids in a steady state reaction at high pressure and high temperature in a stirred reactor in the presence of a free-radical initiator are, Canadian Pat. No. 655,298 (and its U.S. counterpart U.S. Pat. No. 4,351,931); U.S. Pat. Nos. 3,239,370; 3,520,861; 3,658,741; 3,884,857; 3,988,509; 4,248,990; 4,252,924; 4,417,035 and 4,599,392.
U.S. Pat. No. 3,239,370 discloses a random copolymerization of ethylene with an unsaturated carboxylic acid (e.g., acrylic acid) in a stirred autoclave reactor operated at 16,000 psi. and 210.degree. C. using a peroxide initiator, the so-formed copolymer being particularly useful as a coating for non-metallic substrates.
U.S. Pat. No. 3,520,861 discloses a substantially homogeneous, compositionally uniform, random copolymer of ethylene/unsaturated acid (e.g., acrylic acid, methacrylic acid or crotonic acid) prepared in a continuous manner in a stirred autoclave reactor at high pressure and elevated temperature, using a free-radical initiator (such as a peroxide). The temperature of the polymerization is disclosed as being in the range of about 120.degree. C. to about 300.degree. C., preferably about 150.degree. C. to about 250.degree. C. The pressure of the polymerization is disclosed as being in the range of at least 1000, preferably between about 1000-3000 atmospheres, esp. between 1100-1900 atmospheres. The process is preferably operated using a recycle of the unreacted ethylene and comonomer to which make-up quantities of ethylene and comonomer are added prior to reinjection into the stirred autoclave.
Canadian Pat. No. 655,298 and its U.S. counterpart (U.S. Pat. No. 4,351,931) discloses homogeneous, compositionally uniform, random copolymers of ethylene and unsaturated carboxylic acids (e.g., acrylic acid) wherein said copolymer comprises at least about 90% by weight of ethylene with a melt index of 0.01 to 30 g/10 minutes. The copolymers are prepared in a well-stirred reactor at a pressure of at least 1000 atmosphere, at 90.degree.-280.degree. C., using a free-radical initiator, while maintaining the ratio of monomers (ethylene/acid) in the range of 10,000/1 to 50/1 by weight, the process being performed continuously by feeding monomers in, while removing reaction mixture, and maintaining a constant reaction environment.
U.S. Pat. No. 3,658,741 discloses homogeneous copolymers of ethylene and unsaturated carboxylic acids and esters, prepared in the presence of a telogen or chain-transfer agent, a free-radical catalyst, a temperature between 100.degree. and 300.degree. C. and a pressure between 100 and 1000 atmospheres, using turbulent agitation; the reaction is said to occur in the vapor phase and produces very low molecular weight emulsifiable copolymers i.e., waxes.
U.S. Pat. Nos. 3,884,857 and 3,988,509 disclose the preparation of copolymers, such as ethylene/acrylic acid copolymers, in a continuous, high pressure, free-radical polymerization process, at 100.degree.-250.degree. C. and 1000-2500 atmospheres of pressure.
U.S. Pat. No. 4,248,990 discloses copolymers, e.g., ethylene/acrylic acid copolymers which are said to be distinguished over the random copolymers of Canadian Pat. No. 655,298 and of U.S. Pat. No. 3,520,861 by virtue of being non-random. This non-randomness is said to be the result of operating the steady state, high pressure, stirred reactor at a pressure of from 0 to about 500 psi. above, and at a temperature of from 0.degree.-15.degree. C. above, that needed to maintain a single-phase reaction mixture at the given concentration of copolymer in the reaction mixture and at the given comonomer content of the polymer. This patent also teaches that the temperature and pressure for the phase change from 2-phase to single phase, and from non-random to random is dependent on the particular monomers and ratios of monomers being used.
U.S. Pat. No. 4,252,924 discloses the preparation of non-random copolymers, e.g., ethylene/acrylic acid copolymers in at least two constant environment stirred autoclaves in series, each using a single-phase reaction mixture, but where each succeeding autoclave is maintained at a temperature of at least 30.degree. C. above that of the preceding autoclave.
U.S. Pat. No. 4,417,035 discloses copolymers having from 1,500 to 20 MFI (melt flow rate) units as measured at 160.degree. C. under 325 g load, i.e. ASTM D-1238 (160.degree. C./325 g). Such polymers have melt flow rates of more than 350 g/10 minutes when measured in accordance with ASTM D-1238 (190.degree. C./2160 g).
U.S. Pat. No. 4,599,392 discloses homogeneous, random interpolymers, e.g., ethylene/acrylic acid copolymers, prepared in a constant environment, steady state reaction mixture using a well-stirred autoclave reactor at continuous single-phase operation, that necessitates elevated temperatures and pressures wherein the phase boundary is exceeded and a molecular weight distribution (MWD) boundary is reached or surpassed. Narrow molecular weight distributions are said to be attainable at elevated temperatures and pressure; however, exact or useful molecular "tailoring" capabilities are largely limited by the position of the phase boundary and the MWD boundary relative to given reaction temperature and pressure capabilities. This patent is incorporated herein by reference.
Whereas the use of telogens or chain-transfer agents to control molecular weights and polymer densities is common in ethylene homopolymerizations, operators of processes involving interpolymerizations of ethylene and unsaturated carboxylic acids are not inclined to employ chain-transfer agents due to the proclivity of ethylene/carboxylic acid interpolymers to undergo beta-scission during the polymerization reaction and thereby form relatively low molecular weight polymers in contradistinction to high pressure low density polyethylene (LDPE) homo-polymerization at the same reaction conditions.
If a chain-transfer agent is also introduced into the interpolymerization reaction, the polymer molecular weight is further lowered and even higher pressures and/or lower temperature are required for additional compensation to produce the targeted high molecular weight interpolymer. However higher pressures dictate more energy and lower temperatures sacrifice conversion (production rate), and thereby in the absence of any recognized benefit, utilization of a chain-transfer agent is deemed unnecessary and futile for ethylene and unsaturated carboxylic acid interpolymerizations.
We have now found, by operation of the present invention, that it is possible to utilize a chain-transfer agent (telogen) in ethylene and unsaturated carboxylic acid interpolymerizations to control molecular weight distributions and long chain branching and achieve unexpected benefits while maintaining high molecular weights.